Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Acoustic touch sensor panels can be formed from a material that conducts ultrasonic waves, such as a plate of glass. Conventional acoustic touch sensor panels typically employ various schemes to detect touch, including a variety of schemes to transmit, for example, ultrasonic waves across a touch surface, e.g., utilizing various configurations of reflective gratings, transducer configurations, etc., and a variety of schemes to gather information from the resulting received waves. However, most conventional acoustic touch sensor panels cannot accurately sense more than one touch on the panel at a time.